Investigations of mechanisms by which tumor cells are able to evade the immunological defenses of the mammalian host will be continued. Specifically, studies of masking of cell surface histocompatibility (H-2) and tumor specific antigens, and the modification of antigenic activity, by large endogenous glycoprotein molecules in murine mammary adenocarcinoma and lymphosarcoma cells will involve immunological, chemical, and biochemical, immunochemical and physical (electron microscopic) experiments.